The present invention relates to an apparatus for measuring orientations of the stem or bow of a vehicle and more particularly to an apparatus for measuring orientations of stem of ship at sea by making use of radio waves transmitted from the Global Positioning System (as referred to GPS radio waves hereafter).
It is necessary to confirm an accurate position of ship at sea in order to sail with safety and ensure that the course of the ship will be fixed for future sailing. Generally, various navigation measurement apparatus is provided on ships to perform the above purpose. Magnetic compasses and Gyro compasses are the most popular measurement apparatus for sailing, both of which belong to measurement devices for navigation by dead reckoning.
An ordinary magnetic compass is based on a principal in which a magnetic needle mounted for free rotational movement indicates the magnetic poles of the earth. However, orientation of the magnetic poles of the earth is relatively offset from the exact north-south direction thereof so that the needle of magnetic compass does not always indicate the accurate north-sourth line. Furthermore, the indication of a magnetic needle sometimes deviates from the accurate north-south direction where a body of a ship or load thereon are made of materials which are susceptible to magnetic force. Thus, the deviation mentioned above necessarily requires a correction for accurate course thereof based on extensive experiences.
On the one hand, a gyro compass is based on a principal in which rotating action of the earth will affect a top rotating at high speed thereby indicating the orientation of axis of the earth. However, there are disadvantages to a gyro compass in that it is necessary to have a periodical maintenance and the life expectancy is not high since the device includes a mass rotating at high speed therein. Furthermore, it is not practical for ships, which are sailing in a pole region, to use either a magnet compass or gyro compass because of the regional characteristics thereof. In addition, a navigation measurement apparatus that makes use of inertia has the disadvantage of being very expensive to install on a ship.
On the other hand, there are some disadvantages where a stem orientation of a ship is determined by the use of artificial satellites. For example, radio waves transmitted from a static satellite can not generally reach a ship which is at sea in the pole region. Also, it is difficult to obtain a stem orientation of a ship which is sailing along the equator by using radio waves from static satellites since they are generally at a zenith so that the horizontal component thereof is too small to calculate the orientation thereof.
Further, it is not preferable to make use of mobile satellites for measurement of the stem orientation of a ship since they do not always belong to the same satellite system and each of them are not always within a proper angle of elevation.